Pesticide Residues in Water

John Unsworth8th April 2010

National governments introduced residue limits and guideline levels for pesticide residues in water when policies were implemented to minimize the contamination of ground water and surface water. Initially, governments mainly focused on drinking water. Contamination of ground water by pesticide residues was for many years generally regarded as unlikely because the soil profile acts as a purifying filter. Residue contamination of surface waters was regarded as transitory because the focus was on the old organochlorine pesticides, which were attached to particulate matter and generally disappeared from clear water. In the early 1980s information had accumulated that some herbicide compounds, which were generally more water-soluble and more widely used than the organochlorines, were being detected in both surface and ground waters. Policies were developed to reduce contamination of ground and surface water and regulatory limits and guideline levels were introduced for residues in drinking water.

Setting regulatory limits for pesticide residues in waters is complex. First the type of water relevant to the proposed limit, e.g., drinking water, reservoir water, lakes and streams, ground water, water for aquaculture, irrigation water, and drinking water for farm animals – see , for example the EU Water FrameWork Directive1, must be defined. Secondly, should a risk-based approach, a “no more than reasonable if good practices are followed” approach, or a combination of the two be adopted? Different approaches will lead to the setting of different maximum limits. A limit based on a risk to human health or to the environment may allow much higher levels of residue in the water than would ever occur in practice. An arbitrarily chosen maximum limit may be economically wasteful in requiring correction of harmless residues that do not meet the standard, while ignoring more hazardous contaminants that are technically not pesticides. An important principle is that the establishment of guideline levels or standards does not imply that the water quality may be allowed to degrade to the recommended levels.

Drinking water standards rely on a variety of criteria, which are difficult to comprehend - even for experts2. When the standards are perceived as a dividing line between safe and unsafe, a drinking water level exceeding the standard level is of great concern to the public. Regulatory limits for pesticide residues in waters should have the following characteristics: definition of the type of water, definition of the residue, a suitable analytical method for the residues, and an explanation of the basis for each limit. Limits may be derived by applying a safety factor to a no-effect-level, or from levels occurring when good practices are followed, or from the detection limit of an analytical method, or directly by legislative decision. Limits have been most commonly developed for drinking water, but values have also been proposed for environmental waters, effluent waters, irrigation waters, and livestock drinking waters. The contamination of ground water is of concern because it may be used as drinking water and may act as a source of contamination for surface waters. Most commonly, drinking water standards have been applied to ground water.

Various countries or international bodies have published guidelines for setting water quality standards e.g. the World Health Organisation (WHO)3, the European Union4, Canada5, Australia6, USA7.

The World Health Organization’s guideline values for drinking water for those pesticides exhibiting threshold toxicity effects are derived from the tolerable daily intake (TDI) or acceptable daily intake (ADI) by assuming daily consumption of 2 litres of water by a 60-kg adult. For pesticides that are highly persistent, have a high bioaccumulation potential, and are often found in food, only 1 % of the TDI is allocated to drinking water. In other cases, a default value of 10 % TDI is allocated to drinking water. National governments often follow the same procedure in principle, but the details are different. Canadian pesticide residue guidelines for irrigation water8 take into account the phytotoxicity of the residues to sensitive crops. For non-herbicides or non-phytotoxic residues, an additional basis for guidelines would be the accumulation of residues in crops. Residues of a systemic pesticide in irrigation water could be taken up to produce a residue level in the crop exceeding the maximum residue limit (MRL). The maximum guideline limit would be set so that residues in the crop would not exceed the MRL. Canadian livestock water-quality guidelines are derived from animal toxicity studies. An additional concern, as with residues in crops from irrigation water, is the resulting residues in food commodities - in this case in meat, milk, and eggs. Farm animal feeding studies provide information on the relation between residue levels in the animal diet and the resulting residue levels in the animal tissues, milk and eggs. The feeding studies would allow calculation of the maximum residue intake from livestock drinking water before residues in animal commodities exceeded MRLs.

The same terminology may have different meanings in different systems. For example, guideline value (GV) to the WHO means a value calculated from a toxicology parameter, while in
Australia a GV is at or about the analytical limit of determination, or a maximum level that might occur if good practices are followed. In
New Zealand the GV is the concentration where aesthetic significance is influenced. The Australian health value (HV) is conceptually the same as the WHO GV. The
New Zealand maximum acceptable value (MAV) and the Canadian maximum acceptable concentration (MAC) are also conceptually the same as the WHO GV.

Each of the possible ways of defining the residues has its merits. A residue limit in water expressed as the sum of parent and toxicologically relevant transformation products makes sense where it is derived from the ADI. For monitoring purposes, where it is best to keep the residue definition as simple as possible for the sake of practical enforcement and economy, the parent or a marker residue is preferable. It is also possible for parent and degradation products (hydrolysis and photolysis products and metabolites) to become physically separated as the water moves through soil strata, which suggests that separate limits should be set for parent and important degradation products.

An analytical method must be available to measure the residue at a standard or guideline limit designed for surveillance or regulatory enforcement. The specified limit should be no lower than the method LOQ (limit of quantification), which is the lowest concentration where suitable recoveries are achieved (usually mean recoveries of between 70 % and 110 %). In some situations the limit must be set at a level at or below those where relevant biological effects are observed, which may require additional work to ensure that a suitable analytical method is available.

This paper has been taken from “Regulatory Limits for Pesticide Residues in Water”, prepared by the IUPAC Commission on Agrochemicals and the Environment, where further information regarding theguidelines for setting limits on pesticide residues in water can be found9.

References

(1) Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 Establishing a Framework for Community Action in the field of Water Policy, Official Journal of the European Communities L327/1, 22nd December 2000.

(8) Environment
Canada, Canadian Water Quality Guidelines for the Protection of Agriculture Water Uses, Protocols for Deriving Water Quality Guidelines for the Protection of Agriculture Water Uses (Irrigation and Livestock Water), 1999